Hot chamber die casting of semisolids

ABSTRACT

A hot chamber method of die casting material in a semisolid state. The semisolid material has a high viscosity, which can be controlled by controlling the fraction of solid phase and the morphology of the solid phase. By controlling the viscosity of the melt, turbulence and consequent gas entrapment can be minimized or eliminated. Further, shrinkage is substantially reduced, thereby reducing porosity and hot tearing to form stronger, more reliable castings.

FIELD OF THE INVENTION

[0001] This application relates to methods and apparatus for hot chamberdie casting of semisolid materials.

BACKGROUND OF THE INVENTION

[0002] Die casting has traditionally been divided into cold chamberprocesses and hot chamber processes. Hot chamber processes aredistinguished by the fact that the injection cylinder is at leastpartially immersed in the molten metal, and thus is at the sametemperature as the molten metal. Hot chamber die casting is widely usedfor light alloys such as magnesium—and zinc-based alloys, but has notbeen found to be commercially viable for casting aluminum alloys. Thesealloys generally have a higher melting temperature, and thus tend torapidly degrade steel die casters using a hot chamber process.

[0003] Advantages of the hot chamber casting process include higherproductivity, reduced scrap and metal losses, reduced die closingpressures, and reduced die wear. Both hot and cold chamber processes,however, suffer from the disadvantage that it is difficult to producefully sound castings. Liquid metal generally enters the die in aturbulent fashion, entrapping mold gases and forming oxide inclusions inthe finished part. Further, solidification shrinkage produces porosityand sometimes tears in the finished casting. It is an object of thepresent invention to provide a hot-chamber die casting system whichminimizes or eliminates these disadvantages.

SUMMARY OF THE INVENTION

[0004] The present invention supplies a hot chamber method of diecasting material in a semisolid state. The semisolid material has a highviscosity, which can be controlled by controlling the fraction of solidphase and the morphology of the solid phase. By controlling theviscosity of the melt, turbulence and consequent gas entrapment can beminimized or eliminated. Further, shrinkage is substantially reduced,thereby reducing porosity and hot tearing to form stronger, morereliable castings.

[0005] In one aspect, the invention provides a method of die casting, inwhich a semisolid composition is held between its liquidus and solidustemperatures, and agitated to prevent the formation of interconnecteddendritic networks. The composition forms a slurry of solid particles inliquid, which is pumped into a die by an immersed pump. The material isthen cooled to cast it in the die. The material may be, for example, alight alloy such as a magnesium, zinc, or aluminum alloy.

[0006] In another aspect, the invention includes a hot chamber diecaster adapted to cast semisolid materials. The die caster includes acontainer for holding a composition in the semisolid state, and a pumpfor pumping the semisolid material into a die. Agitation means preventthe formation of dendrites, holding the material in a semisolid slurrystate. The agitation means may be, for example, mechanical orelectromagnetic. The caster may be used to cast a variety of lightalloys, such as magnesium, zinc, and aluminum alloys. The pump maycomprise ferrous materials such as stainless steel.

BRIEF DESCRIPTION OF THE DRAWING

[0007] The invention is described with reference to the several figuresof the drawing, in which,

[0008]FIG. 1 is an illustration of a typical hot chamber die castingmachine;

[0009]FIG. 2 is an illustration of one embodiment of a hot chamber diecaster according to the invention; and

[0010]FIG. 3 is an illustration of another embodiment of a hot chamberdie caster according to the invention.

DETAILED DESCRIPTION

[0011]FIG. 1 shows a typical hot chamber die caster 10, such as iscommonly used for casting of magnesium and zinc alloys. The caster workson a “sump pump” principle, using an immersed piston 12 to force moltenmetal into the casting chamber 14.

[0012] A hydraulic cylinder 16 reciprocates the piston 12, within apiston chamber 17 whose end is connected to a gooseneck chamber 18leading to the casting chamber 14. As the piston 12 reaches the top ofits stroke, molten metal 20 flows into the piston chamber 17 and thegooseneck chamber 18 through an aperture 22. When the piston 12 thenmoves down into the chamber 18, it seals the aperture 22 and forcesmolten metal into the casting chamber 14. The casting chamber 14 isdefined by two mold halves 24 and 26. Once the molten metal 20 in thecasting chamber 14 has solidified, mold half 26 is moved to release thecast part. The mold is then closed and another cycle of the system canbe performed. The gooseneck 18 and cylinder head 16 are thuscontinuously exposed to molten metal in this process.

[0013] The semisolid (or rheocasting) process was discovered abouttwenty years ago in the laboratory of one of the present inventors. Itwas found that mechanical stirring of a material between the liquidusand solidus temperatures could break up dendrites, forming a slurry ofspheroidized solid particles in liquid. The viscosity of the materialcan be set to a value in the range of 10⁻¹-10⁸ poise, simply bycontrolling the stirring rate. Detailed descriptions of semisolidprocessing techniques can be found, for example, in U.S. Pat. Nos.3,954,455 and 3,948,650 to Flemings, et al., both of which areincorporated herein by reference. Rheocast castings are generally ofmore uniform strength and of lower porosity than conventional castings.

[0014] The present invention uses semisolid processing to die castmaterials using a hot chamber process. FIG. 2 shows a die casterdesigned to carry out this process. It is similar to the die castershown in FIG. 1, but includes a mechanical stirrer 28 for agitatingsemimolten metal 21. In the embodiment shown, the furnace is providedwith a cover 29 and a pressure inlet 30 to aid in forcing semimoltenmetal 21 through the aperture 22 into the piston chamber 17. Addedpressure is not necessary in standard hot-chamber casting processes,because of the very low viscosity of fully molten metal (typically onthe order of 10⁻² poise). The higher viscosity of the semisolidcompositions of the present invention may make applied pressurepreferable or even essential, depending on the properties of thesemisolid composition and of the caster material.

[0015] The optimum applied pressure for any given embodiment depends onthe solid fraction of the semisolid metal and the speed with which it isdesired to fully fill the piston chamber 17. It is preferred that diecasters according to the invention be able to apply a pressure of atleast 30 psi gauge (i.e., 30 psi above atmospheric pressure). Ifdesired, applied pressure and the viscosity of the semisolid metal canbe adjusted to provide a relatively high fill rate while minimizing theturbulence of flow into the casting chamber 14.

[0016] A temperature controller maintains the melt 20 within arelatively narrow temperature range, in order to ensure that it staysbetween the liquidus and solidus temperatures. For example, the liquidusand solidus temperatures differ by about 120° C. for Mg-8%/Al-1% Zn, acommon magnesium casting alloy. Known process-control techniques can beused to ensure that the metal temperature and viscosity are kept withinacceptable limits.

[0017]FIG. 3 depicts an embodiment of the die caster related to that ofFIG. 2, but using electromagnetic, rather than mechanical, stirringmeans. A set of coils 32 is provided for heating and stirring thesemimolten metal 20. The use of electromagnetic stirring and heating maysimplify the application of pressure, since the coils 32 do not need tobe placed within the semimolten metal 21.

[0018] Hot chamber die casting of semisolid materials offers severaladvantages. The lower temperatures required may provide reduced energycosts and reduced wear rates for casters, and may expand the list ofmaterials which can be inexpensively die cast by the hot chamber method.Further, the increased viscosity of the melt reduces turbulence as themelt enters the die. Reduced turbulence leads to minimal gas entrapmentand thus to a reduced concentration of oxide inclusions. In addition,the shrinkage from the semisolid to the solid state is substantiallyless than that from the fully liquid to the solid state. Thus, shrinkageporosity and hot tearing are reduced in the present process, allowingsimpler and less expensive mold designs to be used.

[0019] Other embodiments of the invention will be apparent to thoseskilled in the art from a consideration of the specification or practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with the true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method of die casting, comprising holding acomposition at a temperature between the liquidus temperature and thesolidus temperature of the composition; agitating the composition toform a slurry of primary solids comprising discrete solid particles inliquid while preventing the formation of interconnected solid dendriticnetworks; pumping the composition into a die with a pump at leastpartially immersed in the composition; and solidifying the material inthe die.
 2. The method of claim 1 , wherein agitating is accomplished bymechanical stirring.
 3. The method of claim 1 , wherein agitating isaccomplished by electromagnetic stirring.
 4. The method of claim 1 ,wherein the composition comprises a majority component of a metalselected from the group consisting of magnesium, zinc, and aluminum. 5.The method of claim 4 , wherein a surface of the pump in contact withthe composition comprises a ferrous material.
 6. The method of claim 1 ,further comprising applying a pressure greater than atmospheric pressureto the slurry.
 7. A hot-chamber die caster, comprising: a container forholding a semisolid composition between its liquidus and solidustemperatures; means for agitating the semisolid composition to form aslurry of primary solids comprising discrete solid particles in liquidwhile preventing the formation of interconnected solid dendriticnetworks; a die for casting the composition; and a pump, at leastpartially immersed in the semisolid composition and arranged to pump thecomposition into the die.
 8. The hot-chamber die caster of claim 7 ,wherein the agitation means comprise a mechanical agitator.
 9. Thehot-chamber die caster of claim 7 , wherein the agitation means comprisean electromagnetic agitator.
 10. The hot-chamber die caster of claim 7 ,wherein the caster is adapted to cast an alloy comprising a metalselected from the group consisting of aluminum, magnesium, and zinc. 11.The hot-chamber die caster of claim 10 , wherein a surface of the pumpin contact with the semisolid composition comprises a ferrous material.12. The hot-chamber die caster of claim 7 , further comprising apressure inlet for applying a pressure greater than atmospheric pressureto the semisolid composition.